Bohnsack, I think you're on target with the MN4.  If this is the way you have to go to make it work, it beats not going.  You can plan your route to take advantage of the big boat's strengths, ride out the weather if too windy and stick closer to shore to avoid getting caught in the open.  The MN4 has large flat center seats (if renting, double check with your outfitter for the seating layout). Most outfitters will have at least one.  I'd go with mom up front, 10 year old next, the younger ones side by side third seat, you in the stern.  If that doesn't work out, you could put one center sitting on a pack.  Ditto what MM said about mom and front steering strokes.  You should easily be able to fit 5-6 large packs in there too.  Smaller packs work better so the younger ones can be more help on portages.  If fishing is in the cards and it's less than calm, throw a few packs in sitting low to help stability and lower the profile a bit.  Good luck with the planning.

pd